Traditionally, incandescent and fluorescent lights have been the primary source of artificial illumination. However, significant advances in the light emitting diode (LED) technology have made the light output of LEDs not only for indicator applications but also sufficient for general illumination.
LEDs are a type of semiconductor device requiring direct current (DC) electricity input source for operation. Conventional LED lighting unit comprises a LED driver power supply, a dimming module, a dimming controller panel, and arrays of LEDs. The LED driver power supply takes an external electricity power input source and convert to a constant direct current output, feeding the dimming module and in turn the LEDs. For land and building use, the external power source is usually an alternative current electricity power source. In such conventional LED lighting unit, one limitation is that the choice of LED driver power supply used must be carefully matched with the capacity of the LEDs, as the maximum current driving the LEDs is fixed by the LED driver power supply based on its type and capacity.
One common type of dimming module is the low frequency Pulse-width Modulator (PWM). The average value of current fed to the load LEDs is controlled by turning a switch between the power input and load on and off at a fast pace. The longer the switch is on compared to the off periods, the higher the current supplied to the load is. Thus, the current supplied to the LEDs is Imin≦ILED≦IDriver, where ILED is the current supplied to the LED; IDriver is the direct current output of the LED driver power supply; and Imin is minimum current that the PWM dimming module can output. The switching frequency of this type of PWM dimming module is usually controlled by a separate dimming controller panel. One disadvantage of this arrangement is the complicated wiring and installation. Another disadvantage is the relatively high cost associated with a separate dimming controller panel.
U.S. Patent Application Publication No. 2007/0097043 discloses a LED driver aims to improve the efficiency of LED by adjusting the feeding current in correlation with the LED temperature. In this disclosure, although a control circuit is used to adjust the feeding current to the LED, this control circuit is to maintain a constant chromaticity and/or luminosity of the LED and is not meant to be used to adjust the brightness of the LED.
U.S. Pat. No. 6,586,890 discloses a driver circuit for LEDs using pulse width modulation (PWM). It also uses current feedback to adjust power to the LEDs and provides a full light and a dim mode. The disclosed driver circuit aims to provide good regulation and efficiency while the current feedback is to maintain operation at the LEDs' nominal current. But the disclosed system is not an integrated system providing both a constant D/C and user-adjustable dimming functionality.